1. Field of the Invention
The present invention relates to a discharge lamp comprising a translucent ceramic tube body dosed with a metallic halide. More particularly, the present invention relates to a discharge lamp having an enhanced output.
2. Description of the Related Art
The arc tube of this type of a discharge lamp comprises a narrow tube portion at both ends thereof. The narrow tube portion is formed by tapering the both ends of a tube body made of a translucent ceramic such as polycrystalline alumina. An electricity-introducing member connected to electrodes is inserted in the narrow tube portion and sealed with a sealing glass.
The reason why a narrow tube portion is formed at both ends is that the thermal capacity of the arc tube is reduced to enhance the lamp efficiency. Another reason is that the size of the sealed portion is reduced to enhance the reliability of airtightness.
However, it has been extremely difficult for this type of a discharge lamp to raise the output of its arc tube to a value as high as rated power of 200 W or more. The reason for this difficulty will be as follows.
In order to enhance the lamp efficiency, the thermal capacity of the arc tube must be reduced. To this end, the length of the narrow tube portion must decrease to reduce the thermal capacity.
However, as the length of the narrow tube portion decreases, the central part of the arc tube, the temperature of which has been raised due to the enhancement of output, and the sealed portion are close to each other, making the temperature of the sealed portion considerably high. This can easily cause the reaction of the sealing glass of the sealed portion with a metallic halide as an enclosure. This also causes the increase of thermal stress developed on the various members constituting the sealed portion, causing the production of a gap between the electricity-introducing member and the sealing glass. Accordingly, as the length of the narrow tube portion decreases, the sealed portion is subject to loss of airtightness leading to gas leak.
On the contrary, of course, as the length of the narrow tube portion increases, the temperature of the sealed portion lowers, making it possible to sufficiently maintain its airtightness. However, the resulting increase of thermal capacity of the arc tube causes the drop of lamp efficiency.
It is therefore an object of the invention to provide a discharge lamp including a translucent alumina tube and having an output of from 200 W to 450 W, which has the enhanced reliability of airtightness of a sealed portion therein while the lamp efficiency thereof is increased.
It was found in the invention that when L and P satisfy the relationship (P+268.75)/31.25xe2x89xa6Lxe2x89xa6(P+456.25)/31.25 and 200 Wxe2x89xa6Pxe2x89xa6450 W, the reliability of airtightness of the sealed portion can be enhanced while the lamp efficiency is increased. Here, L (mm) is the length of a protrusion from a main tube body of a narrow tube of the discharge lamp and P (W) is the rated power of the discharge lamp.
When the foregoing relationship is satisfied, a central portion of the main tube body, the temperature of which rises during lighting of the lamp, and the sealed portion can be sufficiently separated from each other. Further, even during lighting of the lamp, the temperature of the sealed portion can be kept low. Accordingly, the reaction of the sealing glass of the sealed portion with the enclosure and the development of thermal stress on the various members constituting the sealed portion can be suppressed to enhance the airtightness of the sealed portion, making it possible to prevent leakage.
Further, when the foregoing relationship is satisfied, the rise of thermal capacity can be suppressed to an extent such that the lamp efficiency cannot be lowered.
In the above-mentioned discharge lamp, it is preferable that the temperature of the sealed portion during lighting of the lamp is not higher than 680xc2x0 C.